This invention relates generally to an electromagnetic coupling and, more particularly, to an electromagnetic clutch which may be of the same general type as disclosed in Newton et al U.S. Pat. No. 4,160,498.
In a clutch of this type, a disc-like driving member such as a rotor is disposed in opposing face-to-face relation with a disc-like driven member such as an armature. Both members are made of low reluctance magnetic material (e.g., steel) and are supported to rotate about a common axis. Normally, the driving member is spaced axially from and rotates relative to the driven member. When a multiple turn winding is energized by current supplied from a voltage source, magnetic flux threads across the air gap between the two members and attracts the members axially into engagement. The magnetic flux also couples the members rotatably so as to cause the driving member to transmit torque to the driven member.
It is conventional to form the driving and driven members with multiple poles which cause the magnetic flux to zig-zag back and forth several times across the air gap between the two members. The multiple poles usually are formed by providing at least one high magnetic reluctance region in one of the members and at least two high magnetic reluctance regions in the opposing member. Usually, the two high reluctance regions of the one member are spaced radially from opposite sides of the high reluctance region in the opposing member and, as a result, the flux is forced to thread a zig-zag path across the air gap between the two members. The high reluctance regions may be provided either by slotting or grooving the members to form one or more circumferentially extending air gaps therein or by placing circumferentially extending bands of non-magnetic material (e.g., copper) in the members. The magnetic poles of each member are located on radially opposite sides of the high reluctance region or regions thereof.
In some applications, it is desirable for the clutch to engage gradually or in a "soft-start" mode rather than becoming fully and abruptly engaged in a matter of milliseconds. Roll U.S. Pat. No. 4,567,975 discloses a clutch with soft-start capability but that clutch requires a special electronic control for reducing the magnitude of the current flowing through the winding during start-up and for then gradually increasing the current to achieve full torque. When the current is at lower magnitudes, the driven member slips relative to the driving member to enable the clutch to reach full engagement gradually rather than abruptly.